Integrated circuit fabrication utilizes photolithographic processes which use photomasks or reticles and an associated light source to project circuit images onto silicon wafers. A high production yield is contingent on having defect-free masks and reticles. Since it is inevitable that defects will occur in the mask, these defects have to be found and repaired prior to using the mask.
Automated mask inspection systems have existed for over 20 years. The earliest such system, the Bell Telephone Laboratories AMIS system (John Bruning et al., “An Automated Mask Inspection System—AMIS”, IEEE Transactions on Electron Devices, Vol. ED-22, No. 7 Jul. 1971, pp 487 to 495), used a laser that scanned the mask. Subsequent systems used a linear sensor to inspect an image projected by the mask, such as described by Levy et al. (U.S. Pat. No. 4,247,203, “Automatic Photomask Inspection System and Apparatus”). Such a technology teaches die-to-die inspection, i.e., inspection of two adjacent dice by comparing them to each other.
As the complexity of integrated circuits has increased, so have the demands on the inspection process. The needs for resolving smaller defects and for inspecting larger areas have resulted in much greater speed requirements, in terms of number of picture elements per second processed. Numerous improvements have been made in an attempt to keep pace with these increased demands.
Another force driving the development of improved inspection techniques is the emergence of phase shift mask technology. With this technology it will be possible to print finer line widths, down to 0.18 micrometers (μm) or less. Typical examples of this technology are described by Burn J. Lin, “Phase-Shifting and Other Challenges in Optical Mask Technology”, Proceedings of the 10th Annual Symposium on Microlithography, SPIE, The International Society of Optical Engineering, Vol. 1496, pages 54 to 79.
Photomasks are used in the semiconductor manufacturing industry for the purpose of transferring photolithographic patterns onto a substrate such as silicon, gallium arsenide, or the like, during the manufacture of integrated circuits. The photomask is typically composed of a polished transparent substrate, such as a fused quartz plate, on which a thin patterned opaque layer, consisting of figures, has been deposited on one surface. Typically, the patterned opaque layer is chromium with a thickness of 800 to 1200 angstroms. This layer may have a light anti-reflection coating deposited on one or both surfaces of the chromium. In order to produce functioning integrated circuits at a high yield rate, the photomasks need to be free of defects. A defect is defined here as any unintended marring of the intended photolithographic pattern. Such defects can include particles, pits, bumps, scratches, and other like blemishes in an inspection surface. Such defects can be caused during the manufacture of the photomask or as a result of the use of the photomask. Defects can be due to, and not limited to, a portion of the opaque layer being absent from an area of the photolithographic pattern where it is intended to be present, a portion of the opaque layer being present in an area of the photolithographic pattern where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or opaque layer. During the manufacture of photomasks, automated inspection of the photomask is performed in order to ensure a freedom from the aforementioned defects.
Certain apparatuses have been constructed that separately measure either reflected or transmitted light and use both to classify defects. Both types of light are valuable for detecting different types of defects. However, such devices do not commonly detect light that is reflected and transmitted simultaneously from the same point on the inspection surface. In particular, such devices do not make use of detector arrays to capitalize on the advantages of such techniques. Additionally, such devices require the use of one or more discrete detector elements to individually and separately detect reflected light and transmitted light.
Embodiments of the present invention can be used to overcome these and other limitations of the existing art.